The present invention relates to a tension control method for a nutrunner.
Heretofore, a variety of methods such as a torque method, an angle method and a yield point method have been employed as a control method of screw fastening. It is basically important for screw-fastening to stably control the bolt tension of the fastener. In other words, it is necessary that a member to be fastened be tightened by a bolt with a fixed fastener force, i.e. a fixed bolt tension. In screw fastening, however, since there exists a frictional force between the bearing surface of the bolt head and the member to be fastened and between the fastening torque and the bolt tension of the threaded fastener, it is very difficult to stably establish a desired relationship between the bolt tension and the fastening torque. In other words, even if the screw fastening is stopped at a fixed fastening torque, a fixed bolt tension cannot be obtained. The bolt tension can be accurately detected by mounting on the bolt shank a guage which detects the elongation of the bolt. This method is enough for experimental purposes, but for mass-production use, it is almost impossible in terms of costs. In recent years there have been proposed a method which emits sound waves to the head of the threaded portion from a high-frequency sound wave generator mounted in close contact with the bolt head, measures the frequency of the reflected waves and calculates the bolt tension from elongation of the bolt after fastening, and a method which magnetically detects distortion of the bolt head at the time of fastening and calculates the bolt tension from the detected distortion. Each of these methods converts the displacement of the bolt itself to the bolt tension and calls for precise working of the distortion detecting portion of the bolt or adding to the nutrunner a complex sensor for detecting the distortion of the bolt, and hence has not been put to practical use up to now.